The final sentence could be revised to present a stronger conclusion by emphasizing the complexity of Buck's loyalty in relation to his instincts. A stronger version might be:
"Ultimately, while Buck's loyalty to Thornton anchors him in the human world, it becomes a poignant struggle against the primal call of the wild that beckons him back to his true essence."
This revision highlights both the strength of Buck's loyalty and the undeniable pull of his instincts, presenting a more nuanced conclusion.